Value Analysis of Waste Heat Utilization: A Systematic Interpretation from Thermal Principles to Energy-Saving Data

Within industrial production systems, the efficiency of thermal energy use directly determines energy costs and carbon emission levels. For various heating, drying, evaporation, and calcination equipment using gas or fuel oil as the heat source, although combustion efficiency can exceed 85%,the overall system thermal efficiency is often less than 60%The reason is that high-temperature exhaust gas carries a large amount of unused sensible and latent heat, and the direct emission of this heat is one of the most significant sources of waste in industrial energy consumption.

I. Energy Characteristics of Exhaust Waste Heat
The heat contained in industrial exhaust gas mainly includes two parts:

1. Sensible Heat— directly related to exhaust-gas temperature, mainly originating from combustion products.

2. Latent Heat— the condensation heat of water vapor in the exhaust gas, usually accounting for 15%–30% of the total heat.

For example, in textile stentering machines, coating ovens, or food-drying equipment, the exhaust-gas temperature is often150℃~250℃and, calculated at an air volume of 5000 Nm³/h, the thermal power carried by the exhaust gas is approximately250~400kWIf 50% of this heat is recovered through a heat exchanger, it is equivalent to saving about 20–30 Nm³ of natural gas per hour.

II. Principle of Exhaust Waste-Heat Recovery
The core of exhaust waste-heat recovery isheat exchangeHigh-temperature exhaust gas flows through a heat exchanger and transfers energy to the cold air or water entering the system. Common forms include:

• Plate heat exchangersuitable for heat recovery between clean gas and fresh air, with thermal efficiency up to 65%–75%;

• Finned-tube heat exchangerused in occasions with a small amount of dust in the exhaust gas, with efficiency generally 55%–65%;

• Heat-pipe heat exchangerperforms stably under high-temperature and dusty conditions, with heat-exchange efficiency 60%–70%.

The heat-recovery rate is calculated as follows:

$$η = \frac{t_2 – t_1}{T_1 – t_1} \times 100\%$$

in:

• $T_1$exhaust gas inlet temperature

• $t_1$cold-end air inlet temperature

• $t_2$cold-end air outlet temperature

When the exhaust-gas temperature is 200°C and the cold air is 25°C, if the cold-end air temperature rises to 120°C after heat recovery, the heat-recovery efficiency is approximately 63%.

III. Energy-Saving Effect and Economic-Value Analysis
Take a textile stentering machine as an example:

• Exhaust-gas temperature: 180°C

• Exhaust volume: 6000 Nm³/h

• Recovery efficiency: 60%

• Operating time: 6000 h/year
  After conversion, the annual recoverable heat is approximately:
  Q = 6000 × 1.29 × (180−90) × 1.005 × 0.6 = 419 MJ/h ≈ 116.4 kW
  Equivalent natural-gas saving:about 100,000 Nm³/year。

At a natural-gas price of 3.0 yuan/Nm³, the annual cost saving is approximatelymore than 300,000 yuanwith equipment investment of about 200,000–250,000 yuan,and a payback period of less than 1 year。

IV. Deeper Value: Latent-Heat Recovery and Flue-Gas Plume Abatement
Traditional systems mostly recover sensible heat; ifcondensing heat exchangerorheat-pump waste-heat recovery systemis introduced, the condensation latent heat of water vapor in the exhaust gas can be further utilized, raising the heat-recovery rate to80%~90%。
In addition, by lowering the exhaust temperature below the dew point, the exhaust-gas "white plume" phenomenon can be reduced, achievingdual benefits of energy saving and environmental protection。

V. Summary
From a thermodynamic perspective, exhaust waste heat is one of the easiest-to-recover and fastest-payback industrial energy-saving resources.

• For high-temperature processes, 30%–40% gas savings can be achieved;

• for medium-temperature processes, the heat-recovery rate is typically 25%–35%;

• for composite systems incorporating condensation or heat pumps, the energy-efficiency improvement can exceed 50%.

The utilization of exhaust waste heat is not only an energy-saving measure but also an important part of an enterprise's green-manufacturing system. It returns originally wasted energy to the production process, turning "waste heat" into real "wealth."